Electrical (galvanic) isolation between devices can be provided with an isolator with an isolation barrier is between input circuitry and output circuitry. The input circuitry can be referenced to a first ground and the output circuitry can be referenced to a different, second ground, which is galvanically isolated from the first ground such that there is no current between them.
In addition to providing for isolated transfer of an information signal, such devices typically also have input and output circuitry to be powered by power supplies that are isolated from each other. The power supplies can be provided, for example, with two separate power supplies having different ground, or by providing an isolated DC-DC converter with discrete transformers to derive power for one side of the barrier from power supplied to the other side of the barrier.
An example of a full-bridge forward DC-DC converter is shown in FIG. 1. A converter 100 has switching transistors MP1, MP2, MN1, and MN2 driving a transformer TR1. The four switching transistors can be implemented in all PMOS or all NMOS type. In typical operation, first the transistors MP1 and MN2 are on for a time interval DT (0<D<1); then transistors MN1 and MN2 are on for a time interval (1−D)T, where T represents half the period of a cycle. Next, transistors MP2 and MN1 are on for a time duration of DT; and transistors MP1 and MP2 are on for the duration of the cycle. The voltage or power transfer is controlled by the variable D, as power is transferred only during the two DT periods.
During the first DT interval when transistors MP1 and MN2 are closed (on), current is provided through the primary winding 102 of transformer TR1 and induced in secondary winding 104 for delivery to a rectifier 106, filter 108 and a load (not shown) which is connected between the output terminal V(OUT) and an output-side ground GNDB (which is distinguished from the input-side ground GNDA). Current is also drawn to charge the magnetizing inductance of the transformer. This magnetizing inductance gets discharged in the second DT interval when transistors MP2 and MN1 are turned on.
To produce a small isolator, micro-transformers can be used. As used here, a “micro-transformer” means a small transformer in which at least one winding is formed using planar fabrication methods, including but not limited to semiconductor techniques, and preferably in a way that facilitates interconnection with other circuit elements on the same or similar substrate. A planar winding can be formed over (on or above) a silicon substrate, or on a printed circuit board (PCB) or other material. A micro-transformer is said to be “on-chip” if the windings are both formed over a semiconductor substrate, potentially in contact with or spaced from the substrate. Examples of on-chip micro-transformers, and particularly “air-core” micro-transformers, are shown in commonly assigned U.S. Pat. No. 6,291,907 and U.S. patent application Ser. No. 10/214,883, filed Aug. 8, 2002, and published as publication no. 2003/0042571, both of which are incorporated by reference herein in their entireties. Micro-transformers typically have small inductance (L) and high series resistance (R), so they have small L/R values. The interval DT should be shorter than L/R, or else the transformers will get current saturated and lose efficiency because of the voltage drop across series resistance R. If a filtering inductor LF is also formed as a micro-inductor, further efficiency can be lost due to high series resistance. Large filter inductance can be difficult to obtain with a micro-transformer, thereby encouraging the use of a high value of a filter capacitor C2 to minimize ripple on the converter output. Use of a large filter capacitors is generally inconsistent with a goal of making a small isolators.
In order to use micro-transformers, high switching frequencies are used to drive the transistor switches, in some devices with resonant switching. But as the frequency gets high and DT gets small, the control circuitry can become more complex and difficult.